1. Field of the invention
The present invention concerns a position-controlled actuator comprising a motor supplied with power by a power supply circuit controlled at least by an error signal, a speed reducer converting rotary movement of said motor into linear movement over a given travel, an output sensor indicating the position of the actuator and an error signal computation circuit comparing a commanded position of the actuator with a computed position.
The invention applies in particular to autopilot control of an aircraft, especially a helicopter, to control the position of the aircraft in space.
2. Description of the prior art
In a prior art position-controlled actuator as described above the motor is a commutator-type direct current motor driving a speed reducer comprising a torque limiter and a lead screw type gearbox.
The output sensor is a pair of potentiometers coupled to the output rod which deliver a signal proportional to its elongation, representing the calculated real position of the actuator. This signal is applied to the error signal computation circuit which, if the calculated actuator position is different than the commanded position, supplies a non-null error signal to a power amplifier which energizes the motor until the error signal is cancelled out, at which the time the calculated actuator position is coincident with the commanded value.
Generally speaking, the control and power circuits of the actuator described above are incorporated into the autopilot computer, for example, and in any event are external to the casing containing the mechanical parts of the actuator.
The prior art actuator has a number of drawbacks. With reference to the actuator itself, the commutator-type motor and the potentiometer are subject to friction wear of the electrical contacts (motor brushes and potentiometer cursors). This limits the service life of the actuator and requires frequent dismounting to replace the worn parts. Likewise the torque limiter, beneficial with respect to abutment engagement at maximum speed, is also subject to friction wear. With regard to the control system, the error signal computation circuit or control amplifier is remote from the actuator, implying a large number of connecting wires between the amplifier and the actuator (two motor wires and three wires per potentiometer). Also, the current drawn by the motor and supplied by the power amplifier flows in long wires with the risk of radiated interference affecting nearby wiring and equipment.
One object of the present invention is to overcome the technical problem of providing a position-controlled actuator as described above in which the components subject to friction wear are eliminated so as to increase the service life of the actuator and reduce its maintenance requirements.
Another technical problem to be overcome is that of reducing the number of connecting cables between the autopilot computer and the actuator.